1. Field of the Invention
This invention relates to a cathode structure mounted in an electron gun of a picture tube to emit thermions, and more particularly an impregnated pellet for a cathode structure, which is impregnated with electron emission material, and a method of producing the pellet.
2. Description of the Prior Art
Generally, a cathode structure comprises an impregnated pellet 11 composed of heat-resisting metal and thermion emission material and disposed on the top of a cathode sleeve 12 with a cathode heater 13 mounted therein, as shown in FIG. 1 of the accompanying drawings. The impregnated pellets have been used in the past in an oscilloscope and the like requiring a cathode current and at present are also applied to electron tubes such as picture tubes which tend to be large-sized and require high precision and minuteness. Particularly, the impregnated pellet applied to the cathode structure of the picture tube is an important component having a great influence upon the quality and service life of a product.
As shown in FIGS. 2a and 2b, a prior method of producing the impregnated pellet 11 applied to the cathode structure of the picture tube comprises the steps of preparing a heat-resisting, porous sintered body 11a having a plurality of internal and external voids 15,16 by pressing and sintering high temperature and heat-resisting metal powder such as tungsten (W); impregnating the voids 15,16 of the porous sintered body 11a with a molten working material such as copper, plastics, etc., and then shaping the impregnated sintered body into an element in the form of a coin having an outer diameter of about 1.5 mm and a thickness of 0.4 mm by machining the sintered body to conform to the area of the top of the cathode sleeve 12(FIG. 1); clearing the voids 15,16 of the copper or plastic material through evaporation of the material by high temperature heating, or dissolution of the material by a chemical treatment; removing the remaining metal fragments on the surface of the pellet produced during the machining work by using fine power and high pressure gas; and impregnating the voids 15,16 of the pellet with molten electron emission material such as barium oxide(BaO), calcium oxide(CaO), alumina(Al.sub.2 O.sub.3), etc., at a high temperature atmosphere. Then, after the impregnated pellet 11 thus produced is subjected to a surface treatment and bonded porous pellet is coated with metal such as osmium(Os), ruthenium(Ru), etc., to lower the work function of the cathode structure.
The impregnated pellet must have a smooth surface so as to facilitate emission of the electrons from the electron emission material 14 in the voids 15,16 of the pellet during the operation. the prior impregnated pellet produced through the steps as set forth above is however disadvantageous in that since fine powder and high pressure gas are used to remove the melts which remain on the surface of the pellet after the impregnation of the voids 15,16 with the molten electron emission material to deteriorate the smoothness of the surface, the manufacturing process is increased, resulting in lower productivity and higher manufacturing cost. In addition, since the internal voids 15 of the porous sintered body 11a are isolated from the exterior, only the external voids 16 open to the exterior are impregnated with the electron emission material in the external void is shallow and the distribution of the impregnation areas on the surface of the pellet and the size of each impregnation area are irregular and ununiform. As a result, during the heating operation of the heater of the cathode structure, emission of the thermions from the surface of the pellet may be effected ununiformly and the electron emission material may be exhausted rapidly, thereby to shorten the service life of the product.